Ferroelectric Materials and Their Applications 0444883541, 9780444883544

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Ferroelectric Materials and Their Applications

Yuhuan Xu University of California Los Angeles, CA, USA

1991 NORTH-HOLLAND AMSTERDAM LONDON NEW YORK · TOKYO

© Elsevier Science Publishers B.V., 1991 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 211, 1000 AE Amsterdam, The Netherlands. Special regulations for readers in the USA: This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtainedfrom the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. ISBN: 0 444 88354 1

Published by: North-Holland Elsevier Science Publishers B.V. P.O. Box 211 1000 AE Amsterdam The Netherlands Sole distributors for the USA and Canada: Elsevier Science Publishing Company, Inc. 655 Avenue of the Americas New York, NY 10010 USA

Library of Congress Cataloging-in-Publication Data Xu, Yuhuan, 1938Ferroelectric materials and their applications/Yuhuan Xu. p. cm. Includes bibliographical references and index ISBN 0-444-88354-1 (alk. paper) 1. Ferroelectricity. I. Title. Q0596.5.X8 1991 91-8697 537'.2448-dc20 CIP Printed on acid-free paper Printed in The Netherlands

Foreword by L.E. Cross The ferroelectric community is in general not well served with good practical books that reach to the current 'state-of-the-art' in the subject. Books of the type which delineate for different ferroelectric materials, why you might be interested in them, how you might synthesize them, how you could measure their families of properties, and finally how they could be of use in practical devices. For older 'ferroelectricians' the book by Franko Jona and Gen Shirane on Ferroelectric Crystals was a model of simplicity and lucidity, covering a very wide range of ferroelectric structures and properties neatly characterized under different structure families. Unfortunately, like almost all useful ferroelectric books it is now out of print, and anyway the contents pre-dated soft mode treatments and the veritable 'gold rush' to­ wards applications of unique ferroelectric properties. The book Ferroelectric Materials and Their Applications by Dr. Yuhuan Xu is structural in a manner which is rather similar to the earlier Jona and Shirane book and I believe benefits immensely from this mode of presenta­ tions. Now, however, the treatment is also reinforced with discussion and extensive referencing to an exceedingly broad range of current device applica­ tions. The two first chapters condense a very useful account of the salient characteristic of proper ferroelectrics, and summarize the broad range of experimental techniques required to fully characterize the properties of their most complicated nonlinear, anisotropie, hysteretic, polarizable and deformable dielectrics. Reflecting properly the application 'slant' of the book, major emphasis is placed upon oxide ferroelectrics with structures involving different tungstenbronze structure family of materials and the coverage in the book reflects that interest. Again the major driving force for the crystal studies has been the very good tailorable photo-refractive responses in the bronze crystals. Particularly in this family it is the problems of high quality crystal growth which have held up practical development. The treatment of water soluble ferroelectrics is necessarily brief, and the attention is directed mostly to potassium dihydrogen phosphate (KDP) and triglycine sulphate (TGS) where there are still important practical applica­ tions. The book does explore also a number of the more exotic oxide ferroelectrics, including bismuth titanate, lead germanate and the improper gadolinium molybdate.

V

VI

Foreword by L.E. Cross

The text is rounded off by a short chapter on organic ferroelectrics which briefly summarizes recent work on polyvinylidene difìuoride (PVDF) and discusses some of the developments in piezoelectric ceramics/polymer com­ posites and in ferroelectric liquid crystals. Dr. Yuhuan Xu was in the earliest group of Chinese Visiting Scientists at the ferroelectrics group in the Materials Research Laboratory at The Pennsylvania State University. It has been a real pleasure to keep in contact with his developing ferroelectric work at Zhongshan University in Guangzhou and his more recent activity in the Ceramics Group at University of California, Los Angeles (UCLA). Dr. Xu is a dedicated research scientist and his book on ferroelectric materials brings together a well-ordered comprehensive review of the properties and applications of the most important ferroelectrics, with copious references to the original work in each area. The book will be of real value to graduate students, research and develop­ ment scientists and engineers moving into the ferroelectrics field who will certainly need the background material and the extensive referencing to this rapidly expanding field of application for ferroelectric ceramics and crystals. L. Eric Cross Materials Research Laboratory The Pennsylvania State University USA June 1991

Foreword by S.B. Lang When I visited the laboratory of Professor Yuhuan Xu at Zhongshan University in Guangzhou, People's Republic of China in 1985, he presented me with a Chinese book. When I inquired what it was, he proudly told me that it was entitled Ferroelectric and Piezoelectric Materials and that it was the first book on ferroelectricity ever published in the Chinese language. Although I was unable to read it, many of the figures and tables in its 500 pages were familiar to me and it appeared to cover most of the major ferroelectric materials studied up to 1978 when the book was published. Then Yuhuan told me about the history of the book. He had graduated from Beijing University in 1963 and had taught for three years at Shandong University in Jinan, China. In 1966 the Chinese Cultural Revolution began and Yuhuan could not work. He was to do absolutely nothing. To occupy himself, he began to visit a library and to read scientific journals, most of which were apparently untouched and many of which were still in their original mailing envelopes. Because of his background in solid state physics, he concentrated on solid-state functional materials, especially ferroelectric and piezoelectric solids. He read hundreds of papers and wrote copious notes. In 1970, he was fortunate to be ordered to work in a factory with a workshop which produced piezoelectric ceramic filters and other devices, instead of being sent to the countryside for 're-education' and manual labor. Here he gained practical experience. The Cultural Revolution ended in 1976, and a year later, Yuhuan completed the manuscript on Ferroelectric and Piezoelectric Materials. In 1987, I visited Yuhuan again in Guangzhou. He was then engaged in revising his original Chinese book. I suggested that a version in English would be of great value to workers in ferroelectricity and the concept of Ferroelectric Materials and Their Applications was born. However, the pres­ ent volume is not a translation of the Chinese book, but rather it is a com­ pletely new text covering developments through 1990. Ferroelectric Materials and Their Applications is unique among books on ferroelectricity because of its emphasis on synthesis, properties and applications of materials rather than on ferroelectric phenomena and theories. The first two chapters introduce the basic concepts of ferroelectricity and the methods for measurement of the fundamental dielectric, electromechan­ ical, electrothermal, optical and electrooptic properties. The other seven chapters describe the synthesis, structure, physical properties and applica­ vi!

viii

Foreword by S.B. Lang

tions of a very complete range of ferroelectric materials classified according to families. These include perovskites, lithium tantalate and lithium niobate, tungsten-bronze niobates, water-soluble crystals, miscellaneous inorganic materials, polymers, composites and ferroelectric liquid crystals. Optics and electrooptics are strongly emphasized, reflecting the interests of the author. The book is well illustrated and thoroughly referenced. Each chapter has extensive tables of materials and their properties. There are many references to the Chinese literature which have not previously been available in the English language. This book should be useful both to researchers and to applications engi­ neers, and could well be used as a reference by administrators and managers in industries that produce ferroelectric materials and devices. The level of the book is suitable for advanced undergraduate or beginning graduate students and could serve as a basis for a university course. Thefieldof ferroelectricity is young and is still growing rapidly. Previously unobserved phenomena and new theoretical interpretations are reported frequently. Many applications of the unusual properties of ferroelectric materials are already on the market and many more will undoubtedly appear within the next few years. Ferroelectric Materials and Their Applications should contribute significantly to the evolution of the field. Sidney B. Lang Beer Sheva, Israel January 1991

Preface This book, intended as an introduction to ferroelectric materials for readers working in the fields of applied physics, electrical engineering, laser technique, electronic ceramics fabrication and crystal growth, has two pri­ mary objectives: to present the basic physical properties and structures of ferroelectric materials and to introduce their fabrication methods and their applications in various devices, such as piezoelectric and/or electrostrictive transducers and actuators, pyroelectric infrared detectors, optical integrated circuits, optical data storage and display devices, etc. The ferroelectric materials described in this book include a relatively complete list of practical and promising ferroelectric single crystals, bulk ceramics and thin films: these include perovskite-type, lithium niobate, tung­ sten-bronze-type, water-soluble crystals and other inorganic materials, as well as organic ferroelectrics (polymer, liquid crystals, and composites), as may be seen from the Contents. Basic concepts, principles and methods for measuring physical characteristics of ferroelectric materials are introduced in the first two chapters for those readers who feel ferroelectricity is a new subject. In that sense, this book is suitable not only for professional re­ searchers and engineers but also for students and readers who have limited physical knowledge and yet much interest in ferroelectrics. In the winter of 1987, when the author was teaching at the Department of Physics, Zhongshan University, Canton, China, his good friend, Professor Sidney B. Lang of the Ben Gurion University of the Negev, Israel, suggested that the author should write this book and made the initial contact with the publishers. The author sincerely wishes to acknowledge Professor Sidney B. Lang for all his help and also for writing a preface for this book. The author is profoundly grateful to Professor L. Eric Cross, who offered the author a visiting-scholar position at the Materials Research Laboratory of the Pennsylvania State University, provided him the opportunity to learn more about ferroelectricity at MRL from 1980 to 1981, and gave the author much personal concern through the years. The author expresses his heartfelt appreciation to Professor Cross for the Introduction that he wrote for this book. The author also benefited a great deal from the intuitive discussion with Professor Robert E. Newnham at Pennsylvania State University. The author wishes to thank his friends: Dr. Ren Xu for reading the original manuscript of this book and offering many useful suggestions, Ms. Dora C. Li for formatting and typing all tables, and Mrs. Dongcheng Xie Gong for ix

X

Preface

drafting most of the figures. His thanks also extend to Professor Peter de Châtel, the acquisition editor, and Drs. M. van der Bijl, the desk editor, of Elsevier Science Publishers B.V. for their patient cooperation with a great deal of works in the production of this book. The author also wishes to mention his many Chinese colleagues and friends for their help and dis­ cussions in the field of ferroelectrics. Among them are Professor Zhi-Wen Yin, Professor Xi Yao, Professor Yong-Ling Wang, Professor Ding-Quan Xiao, Professor Jing-De Li, Professor Dang Mo, Professor Zhi-Gang Zhou, Professor Huan-Chu Chen and Professor Wei-Lie Zhong. Special thanks extend to Professor John D. Mackenzie of the Department of Materials Science and Engineering, University of California, Los Angeles, who gave the author many conveniences during the final stage of the manu­ script preparation. The author is grateful to Professor Mackenzie for inviting and supporting him to do research on new ferroelectric thin films since the winter of 1988. The author learned much about the science of ceramics from Professor Mackenzie in the last two years. Yuhuan Xu Los Angeles, California December 1990

1I Introduction: characteristics of ferroelectrics In the past twenty years, many excellent books introducing ferroelectric crystals and explaining ferroelectricity have been published [1-12]. Not much can be said, without sounding redundant, because the principles have already been covered in the previous works. However, for those readers who may not be familiar with ferroelectric materials, this chapter may serve as an introduction. We summarize here some of the characteristics of ferroelectrics as briefly as possible.

7.7. Structural symmetry Structural symmetry of a crystal depends on its lattice structure. The lattice structure is described by Bravais unit cell of the crystal. There correspond to thousands of crystals in nature only thirty-two macroscopic symmetry types (32 point groups). A point group consists of eight symmetry elements (exclud­ ing translation symmetry). These eight symmetry elements are the following: rotation axes: 1 (without rotation), 2 (rotation diad), 3 (rotation triad), 4 (rotation tetrad), 6 (rotation hexad), 4 (rotation-inversion tetrad axis), inversion center i and reflection mirror m. If translation symmetry element is counted in, these symmetry elements comprise 230 microscopic symmetry types (space groups). The thirty-two point groups with their symbols are listed in table 1.1. Structural symmetry of a crystal affects geometrically both structural and physical properties of the crystal, such as dielectric, elastic, piezoelectric, ferroelectric, and nonlinear optical properties, etc. (the details have been summarized in some books, such as in Nye's work [12]). According to Neumann's principle, symmetry elements of all physical properties in a crys­ tal should include all symmetry elements of the point group of this crystal. Thus, if a physical parameter is subjected to a symmetry operation of this crystal, the value of this physical parameter should remain invariant. The thirty-two point groups can be classified as follows: Centrosymmetry type with a symmetry center includes eleven point groups; they are Ï, 2/m, 1

Table 1.1 Symbols of the 32 point groups in crystallography. Crystal system

Triclinic

Monoclinic

Orthorhombic

Tetragonal

International notation

Remarks t

1 1

c,

* + -

2 m(2) 2/m

Ci C,(Clh)

c2h

* + * + -

2mm 222 mmm

C2v D2[V) D2h( yj

* + * -

4 4 42m 422

c* s4

* + * * *

4mm 4/m 4/mmm

h

Schönflies' notation

C^S,)

D2di yd)

c tt

D«

* + -

Crystal system

Trigonal

Hexagonal

Cubic

International notation

Schönflies' notation

Remarks t

* + -

3 3

c3

3m 32 3m

03

6 6 6mm

c6 c3h

* + * * +

6/m 622 6m2 6/mmm

c6h

r

* * *

*"„

♦

0

-

23 43m m3 43 m3m

c 3 ,(s 6 ) C3v *>3d

c 6v

»6 *>«h *>.h

'*' implies that piezoelectric effect may be exhibited and * + ' implies that pyroelectric and ferroelectric effects may be exhibited.

* + * -

Structural symmetry

3

mmm, 4/m, (4/m)mm, 3, 3m, 6/m, (6/m)mm, m3, and m3m. A crystal having a symmetric center does not possess any polarity. Other twenty-one point groups among the 32 point groups do not have any centrosymmetry. They are 1, 2, 222, 4, 422, 32, 6, 622, 23, 3, m, mm2, 4, 42m, 4mm, 3m, 62m, 6mm, 6,43m and 432. A crystal that has these point-group symmetry possesses one or more crystallographically unique direction axes. The two opposite ends of a crystallographically unique direction axis cannot be made to coincide with each other by any symmetry operation of this crystal. All noncentrosymmetric point groups, except the 432 point group, exhibit piezoelectric effect along a unique direction axis. The piezoelectric effect was discovered by Jacques Curie and Pierre Curie in 1880. Although the point group 432 does not have any centrosymmetry, it has other sym­ metry elements whose combination can exclude piezoelectric activity. Among the twenty point groups referred to here that exhibit piezoelectric effect, ten point groups have only one unique direction axis. They are 1, 2, m, mm2,4,4mm, 3, 3m, 6, and 6mm. A crystal having these point-group symmetry has a unique rotation axis, but does not have any mirror perpendicular to this axis. Along a unique rotation axis, the atomic arrangement at one end is different from that at the other (opposite end). Such crystals are called polar crystals since they display spontaneous polarization. A crystal exhibiting spontaneous polarization can be visualized to be composed of negative and positive ions. In a certain temperature range, these ions are at their equilibrium positions, at which the free energy of the crystal is a minimum, and the center of positive charge does not coincide with the center of negative charge. For example, fig. 1.1 shows the crystal structure of

(a)

(b)

Fig. 1.1. The crystal structure of barium titanate (perovskite-type structure), (a) Above the Curie temperature the cell is cubic; (b) below the Curie temperature the structure is tetragonal with Ba 2+ and Ti 4 + ions displaced relative to the O 2 " ions.

4

Introduction: characteristics offerroelectrics

the ferroelectric crystal barium titanate. Above the Curie temperature of 120°C, the prototype crystal structure is cubic, with Ba 2 + ions at the cube corners, 02~ ions at the face centers and Ti 4 + ion at the body center, as shown in fig. 1.1a. Below the Curie temperature, the structure is slightly deformed, with Ba 2 + and Ti 2 + ions displaced relative to the O 2 " ions, thereby creating a dipole, as shown in fig. 1.1b. Thus we may visualize each pair of positive and negative ions as an electric dipole, and the spontaneous polarization (dipole moment per unit volume) as due to an assembly of these dipoles (they point in the same direction). However, we cannot in general attribute the dipole moment to a special pair of ions in the unit cell without making arbitrary assumptions about the interatomic forces. All we can say is that, if there is a given charge density distribution p(r) within a given crystal, the electric dipole moment (with respect to an arbitrary origin)

\\\p(r)rav is different from zero and that the value of this volume integral is independent of the choice of the origin.

7.2. Spontaneous polarization and pyroelectric effect Spontaneous polarization is defined by the value of the dipole moment per unit volume, or by the value of the charge per unit area on the surface perpendicular to the axis of spontaneous polarization. Since electrical prop­ erties are strongly correlated with the crystal structure [13], the axis of spontaneous polarization is usually a crystal axis. Although a crystal with polar axes exhibits piezoelectric effect, it does not necessarily have a spontaneous polarization vector, because the net result of electric moments along all polar axes may be equal to zero. Therefore, only a crystal with a unique polar axis exhibits a spontaneous polarization vector Ps along this axis. In general, this spontaneous polarization cannot directly be measured from the charges on the surfaces of the crystal, because these charges are compensated by external and/or internal carriers carrying an electric current, or by charges on the boundaries of twins. The value of the spontaneous polarization Ps depends on temperature. Figure 1.2 shows the temperature dependence of spontaneous polarization in two typical ferroelec­ tric crystals, BaTi0 3 [14] and TGS (triglycine sulfate) [15]. As temperature changes, a variation of the charge density can be observed on those surfaces of the sample which are perpendicular to the unique polar axis in a crystal

3.5

28 3.0

*^#HL

24